Thin film photovoltaic (PV) modules (also referred to as “solar panels”) based on cadmium telluride (CdTe) paired with cadmium sulfide (CdS) as the photo-reactive components are gaining wide acceptance and interest in the industry. CdTe is a semiconductor material having characteristics particularly suited for conversion of solar energy to electricity. For example, CdTe has an energy bandgap of about 1.45 eV, which enables it to potentially convert more energy from the solar spectrum as compared to lower bandgap semiconductor materials historically used in solar cell applications (e.g., about 1.1 eV for silicon). The junction of the n-type layer and the p-type absorber layer is generally responsible for the generation of electric potential and electric current when the CdTe PV module is exposed to light energy, such as sunlight. Specifically, the cadmium telluride (CdTe) layer and the cadmium sulfide (CdS) form a p-n heterojunction, where the CdTe layer acts as a p-type absorber layer (i.e., a positive, electron accepting layer) and the CdS layer acts as a n-type layer (i.e., a negative, electron donating layer).
A transparent conductive oxide (“TCO”) layer is commonly used between the window glass and the junction forming layers. This TCO layer provides the front electrical contact on one side of the device and is used to collect and carry the electrical charge produced by the cell. Conversely, a back contact layer is provided on the opposite side of the junction forming layers and is used as the opposite contact of the cell. This back contact layer is adjacent to the p-type absorber layer, such as the cadmium telluride layer in a CdTe PV device.
Due to the high work function of CdTe, conventional metal back contacts are not generally viewed as being suitable. Instead, graphite pastes (either undoped or doped, for example with copper or mercury) are widely used as a back contact for CdTe PV cells. However, these graphite-paste back contacts tend to degrade significantly over time, as can be shown via accelerated lifetime testing. This degradation typically manifests itself as a decrease over time in fill factor (FF) and/or open circuit voltage (VOC). The fill factor degradation is often driven by a decrease in shunt resistance (Rsh) and/or an increase in the series resistance (ROC) over time. The degradation of the back contact electrodes undesirably leads to degradation of the solar cell efficiency, on a long-term basis.
A long held understanding of CdTe back contacts made from copper and completed with a conductive paste is that such back contacts need to have some tellurium enriching attribute/mechanism in order to form a good ohmic back contact, either as part of the copper step, as a separate etching process, or by directly depositing a Te-rich layer. Since using a separate etch or depositing a Te-rich layer require an additional process step prior to applying the back contact, it is desirable to use an approach wherein the back contact step creates the Te-rich layer during one of the existing processing steps. The same is true for the copper deposition step; it would be highly advantageous if this step could be combined with some other step in the process.
An early attempt to combine processing steps occurred when Tellurium enrichment was combined with the conductive graphite paste cure. The method involved the generation of acid by products as the paste cured and it was effective at achieving a good ohmic back contact initially, but the process was typically uncontrolled. Later it was discovered that the graphite paste continued to produce acid with time and this caused degradation to the photovoltaic cell. U.S. patent application Ser. No. 13/638,636 of Lucas, et al. filed on Aug. 31, 2012, which is incorporated herein by reference, discloses a method for controlling the Te enrichment during the application and curing of the back contact without loss in cell performance with time. This combination of processing steps reduces manufacture cost and time.
However, further reduction in the number of processing steps is still desired.